Dr. Christophe Caloz
Professor, Electrical Engineering
Canada Research Chair
École Polytechnique de Montréal
Building Lassonde, Office M6025
2500, ch. de Polytechnique
Montréal (Québec), H3T 1J4, Canada
Metamaterials: Past, Present and Future
In the history of humanity, scientific progress has frequently been associated with the discovery of novel substances or materials. Metamaterials represent a recent incarnation of this evolution. As suggested by their prefix “meta”, meaning “beyond” in Greek, metamaterials (artificial materials owing their properties to sub-wavelength but supra-atomic scatterers) even transcend the frontiers of nature, to offer unprecedented properties with far-reaching implications in modern science and technology.
This talk presents some research highlights in electromagnetic metamaterials over the past decade, with emphasis on applications providing performances or functionalities that outperform state-of-the-art technologies. The first part of the talk reviews some history, principles and properties of metamaterials from a global perspective. The second part presents a series of microwave metamaterial applications exploiting these properties, in particular negative refraction, near-zero index propagation, coupling amplification, full-space scanning leakage radiation, and agile temporal and spatial dispersions. This part culminates with the introduction of the concept of radio real-time signal processing, enabled by “phasers” (components with fully designable group delay versus frequency responses), which might play a central role in tomorrow’s radio. The third part introduces magnet-less non-reciprocal metamaterials (MNMs), which have been recently invented and developed in the speaker’s group. While non-reciprocal gyrotropic materials, first reported by Faraday in 1845, have always required a biasing magnet to date, MNMs, which are composed of transistor-loaded rings mimicking electron-spin precession in ferrites, only require a biasing voltage, and are therefore fully compatible with semiconductor technology. This new class of metamaterials might therefore be considered a breakthrough and seem to have a strong potential for commercial electronic and photonic applications. Finally, the talk explores perspectives for next-generation of metamaterials, which will arguably be muli-scale (micro, nano, atomic) and multi-substance (e.g. semiconductors, ferroelectrics, magnetic nanoparticles, multiferroics, carbon nanotubes, graphene, etc.) in nature.
Leaky-Wave Antennas: the Dawn of a New Era!
Leaky-wave antennas (LWAs) have a history of over 70 years. This history started with a patent on a leaky slit waveguide by Hansen in 1940, and the field was then really developed in the late 1950ies and 1960ies by the Brooklyn Polytechnic (now NYU Poly) microwave group, involving Oliner, Tamir and Hessel. Since then, much LWA research has then been carried out by various groups around the world. However, despite some of their unique features, LWAs have been plagued by fundamental issues that have limited their utilization in practical systems. These issues have been recently solved, bringing us to the doorstep of a new area in LAWs.
The unique benefits of LWAs is that they provide high directivity and (frequency or electronic) beam scanning with much smaller form factor, lower cost and higher gain than antenna arrays, as they do not require a complex feeding network. In uniform LWAs, these benefits are annihilated by the restriction of forward-only scanning. Periodic LWAs have been capable of radiating both in forward and backward directions, using leaky space harmonics, since their introduction by Rotman in the late 1950ies. However, their aforementioned LWA benefits have been countered by the collapse of the radiation efficiency at broadside. A definite solution to this persistent issue came in 2002 with the advent of metamaterial Composite Right/Left-Handed (CRLH) LWAs, the first LWAs capable of efficient full-space scanning, which made LWAs potentially superior to arrays. The secrets for this long sought solution were revealed by the groups of D. R. Jackson and of the speaker over the past decade, and then extended to non-metamaterial LWAs: 1) presence of two resonators in the unit cell, 2) closure of the open-stop band by mutual cancellation of the two resonances, 3) satisfaction of a Heaviside-like condition to equalize gain through broadside. Moreover, fundamental relations between the (transverse and longitudinal) symmetries of the periodic unit cell and the LWA properties were recently unveiled by the speaker and collaborators at the University of Duisburg, providing prescriptions completing the broadside radiation ones for most efficient and diverse LWA designs. The talk first overviews historical milestones, explains the physics of LWAs (including their fundamental connection with the Smith-Purcell effect in particle physics) and provides basic electromagnetic tools for their analysis. Next, it presents and illustrates the solution to the broadside radiation issue as well as the unit cell symmetry rules. Finally, it demonstrates a number of novel concepts, structures, systems and applications, including active LWA beam forming, gain enhancement via power recycling, LWA direction-of-arrival estimation, non-reciprocal LWA diplexers, direction diversity enhanced MIMO systems, smart reflectors, graphene-tunable THz antennas, real-time spectrogram analyzers, and vortex beam launchers for orbital angular momentum multiplexing.
Radio Analog Signal Processing for Tomorrow's Radio
Today's exploding demand for faster, more reliable and ubiquitous wireless connectivity poses unprecedented challenges in radio technology. To date, the predominant approach has been to put increasing emphasis on digital signal processing (DSP). However, while offering device compactness and processing flexibility, DSP suffers of fundamental limitations, such as poor performance above the K band, high-cost A/D conversion, low processing speed and high power consumption.
Recently, Radio Analog Signal Processing (R-ASP) has emerged as a novel paradigm to potentially overcome these issues, and hence address the aforementioned challenges. R-ASP processes radio signals in their pristine analog form and in real time, using “phasers”. A phaser is a temporally – and sometimes also spatially – dispersive electromagnetic structure whose group delay is designed so as to exhibit the required (quasi-arbitrary) frequency function to perform a desired operation, such as for instance real-time Fourier transformation. Phasers can be implemented in Bragg-grating, chirped-waveguide, magnetostatic-wave and acoustic-wave technologies. However, much more efficient phasers, based on 2D/3D metamaterial structures and cross-coupled resonator chains, were recently introduced, along with powerful synthesis techniques. These phasers can manipulate the group delay of electromagnetic waves with unprecedented flexibility and precision, and thereby enable a myriad of applications in communication, radar, instrumentation and imaging, with superior performance or/and functionality. This talk presents an overview of R-ASP technology, including dispersion-based processing principles, historical milestones, phasing fundamentals, phaser synthesis, and many applications.
Graphene, a monolayer of carbon atoms arranged in a honeycomb lattice, is the first truly two-dimensional material ever produced by humanity. For this reason, and also due to its exceptional mechanical, thermal, chemical and electronic properties, it won Geim and Novoselv the Nobel Prize in Physics in 2010, only 6 years after their first experimental report on the topic. Since then, this Holy Grail material has spurred huge interest in both the scientific and engineering communities, with over 1000 papers published per month on graphene related topics.
In the area of electronics, during its first lustrum (starting in 2004), graphene research was mostly focused on transport devices (transistors, mixers, switches, etc.), exploiting the high mobility and ambipolarity of the material for higher performance or functionality. However, many researchers have recently directed their attention to the potential of graphene for electromagnetics, due to the discovery of novel phenomena and to the recent availability of large area graphene sheets. One of the key interests in this area graphene’s capability to provide tunable material properties via simple or patterned electrostatic gating. Moreover, fascinating and unprecedented effects occur when the graphene is immersed in a static magnetic field, in which case the electron and hole charge carriers are drawn into cyclotron orbits described by a tensorial conductivity. This area may be called graphene magneto-plasmonics, as graphene essentially behaves as a two-dimensional electron or hole gas. At microwaves, graphene is a transparent conductor whose phase difference between the right-handed and left-handed circularly polarized eigenstates is so significant that electromagnetic waves traveling across it experience giant Faraday rotation, with the possibility of voltage-induced Faraday reversal based on ambipolarity. This phenomenon enables a diversity of unique Faraday devices, such as gyrators, isolators, non-reciprocal radomes and perfect electromagnetic boundaries. At terahertz frequencies, graphene supports tunable surface magneto-plasmons with exotic properties, such as for instance directional concentration and splitting counter-propagating modes, strongly depending on the nature of doping (chemical or electrical). These magneto-plasmons might pave the way for efficient non-reciprocal terahertz electromagnetic components, which are critically missing today. The talk first recalls the fundamentals of graphene and describes some key electronic applications. Next, it introduces magneto-plasmonics, and sequentially presents the microwave Faraday rotation and the terahertz surface magneto-plasmonic phenomenology and applications. Finally, it discusses some multi-scale and multi-physics metamaterial structures involving graphene as a gyrotropic element.
Localized Waves or Molding Electromagnetic Waves
Localized waves (LW), also sometimes called non-diffractive waves or non-diffractive beams, are currently spurring revived interest in the radio and optical communities. A LW is characterized by a strong confinement of the field on a distance that is proportional to the size of the radiating aperture. LWs are solutions to the wave equation. There exist a great diversity of LWs, exhibiting various and fascinating properties. For instance, Bessel LWs exhibit a Bessel constant cross section, vortex LWs feature spiral wave fronts, i.e. carry orbital angular momentum (OAM), which may be applied to OAM multiplexing or particle tweezing, X-LWs are pulsed Bessel waves, of order larger than one and also carrying OAM, that may be designed to produce superluminal centroids, and Airy LWs are accelerated beams, following prescribed curved trajectories.
Until recently, LWs have been mostly restricted to theoretical studies, and have been little exploited in practical applications. However, technological progress in optics technology, where LWs are generally produced by sophisticated spatial light modulators, has brought the area of LWs to the forefront of the stage. At microwave, millimeter-wave and terahertz frequencies, other approaches are required to generate LWs. However, two promising roads – metasurfaces and, more recently, leaky-wave antennas – have been recently opened to meet this new challenge. Moreover, the group of the speaker has introduced two systematic techniques to synthesize metasurfaces producing arbitrary LWs within the limits of the laws of physics: a spatial technique, based on electromagnetic boundary conditions and providing the metasurface susceptility and polarizablities, and a spectral technique, based on the conservation of the total wave momentum and providing the metasurface transfer function in phase and magnitude; the latter includes a reverse propagator technique which allows to control LWs are an arbitrary distance from the source. The talk will first present the fundamentals of LWs and describe some of the most common LWs. It will next introduce the aforementioned spatial and spectral synthesis techniques, and demonstrate their unprecedented capabilities via several examples of exotic waves existing either in the Fresnel region or in the Frauenhofer region of the aperture. Then, a number of metasurface and antenna implementations and applications will be presented. Applications pertaining to communications, security, sensing, imaging, spectroscopy biotechnology, nanotechnology, and astronomy will be presented or discussed.
Christophe Caloz received the Diplôme d'Ingénieur en Électricité and the Ph.D. degree from École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, in 1995 and 2000, respectively. From 2001 to 2004, he was a Postdoctoral Research Fellow at the Microwave Electronics Laboratory, University of California at Los Angeles (UCLA). In June 2004, Dr. Caloz joined École Polytechnique of Montréal, where he is now a Full Professor, the holder of a Canada Research Chair (CRC) in Metamaterials and the head of the Electromagnetics Research Group. He has authored and co-authored over 500 technical conference, letter and journal papers, 12 books and book chapters, and he holds many patents. His works have generated over 11,000 citations. In 2009, he co-founded the company ScisWave, which develops CRLH smart antenna solutions for WiFi. Dr. Caloz received several awards, including the UCLA Chancellor’s Award for Post-doctoral Research in 2004, the MTT-S Outstanding Young Engineer Award in 2007, the E.W.R. Steacie Memorial Fellowship in 2013, the Prix Urgel-Archambault in 2013, and many best paper awards with his students at international conferences. He is an IEEE Fellow. His research interests include all fields of theoretical, computational and technological electromagnetics, with strong emphasis on emergent and multidisciplinary topics, including particularly metamaterials, nanoelectromagnetics, exotic antenna systems and real-time radio.